Amorphous "something" seen at night in the skies over northern California, 5/10/01.

I was with my girlfriend when we had this sighting in Cloverdale, CA, a small town about 85 miles north of San Francisco. She is a teacher and I am an engineer. When I can get out of the Big City (San Diego) I like to look up into the night sky. This night was cloudless, visability excellent. We were taking a leisurely walk when I looked up right at an amorphous, semi-transparent cloud-like light area (not lighted but lighter than the background sky) moving towards us at about 70 degrees in the southern sky. My first thought was that it was a flock of light-colored birds flying at night and I called it to the attention of my girlfriend. It became quickly obvious to both of us it wasn't birds. You could see the stars through it. It varied in its lightness throughout its area (maybe two thumbwidths roundish at arms length), like a group of thin clouds moving together. It wasn't an object or objects. It was amorphous. In the course of about 4 seconds it moved to roughly 70 degrees north (total about 40 degrees) and faded from view. Estimated height 500-1000 feet, speed maybe 200 mph. As this was taking place my mind was trying to put this in a compartment of the known and it completely failed. My girlfriend's response was kind of like "Well that was interesting." My response was more like "What the heck did we just see? That makes no sense. What the heck did we just see?" To reiterate, it was calm, warm, and completely cloudless that night.